Electric Circuits, 9th Edition P7. Designed for use in a one or two-semester Introductory Circuit Analysis or Circuit Theory Course taught in Electrical or Computer Engineering Departments. Electric Circuits 9/e is the most widely used introductory circuits textbook of the past 25 years. As this book has evolved over the years to meet the changing learning styles of students, importantly, the underlying teaching approaches and philosophies remain unchanged. | 36 Circuit Elements resulting from the resistance in the case and coil represents an unwanted or parasitic effect. It drains the dry cells and produces no useful output. Such parasitic effects must be considered or the resulting model may not adequately represent the system. And finally modeling requires approximation. Even for the basic system represented by the flashlight we made simplifying assumptions in developing the circuit model. For example we assumed an ideal switch but in practical switches contact resistance may be high enough to interfere with proper operation of the system. Our model does not predict this behavior. We also assumed that the coiled connector exerts enough pressure to eliminate any contact resistance between the dry cells. Our model does not predict the effect of inadequate pressure. Our use of an ideal voltage source ignores any internal dissipation of energy in the dry cells which might be due to the parasitic heating just mentioned. We could account for this by adding an ideal resistor between the source and the lamp resistor. Our model assumes the internal loss to be negligible. In modeling the flashlight as a circuit we had a basic understanding of and access to the internal components of the system. However sometimes we know only the terminal behavior of a device and must use this information in constructing the model. Example explores such a modeling problem. Example Constructing a Circuit Model Based on Terminal Measurements Hie voltage and current are measured at the terminals of the device illustrated in Fig. a and the values of u and it are tabulated in Fig. b . Construct a circuit model of the device inside the box. - Device V N i A -40 -10 -20 -5 0 0 20 5 40 10 b a Solution Plotting the voltage as a function of the current yields the graph shown in Fig. a . The equation of the line in this figure illustrates that the terminal voltage is directly proportional to the terminal current vt 4it. In terms of
